The use of a vehicle's brake system to slow the vehicle results in high loads being placed upon the brake system, causing brake pad wear and the production of large amounts of heat. In the case of heavier vehicles, various types of auxiliary brakes are used in addition to ordinary wheel brakes. Such auxiliary brakes (for example, an exhaust gas engine brake) allow for prolonged application of the auxiliary brakes without the vehicle's main brakes being used. As a result, the main brakes, which conventionally comprise friction brakes of the disc brake or drum brake type, are prevented from over-heating, thereby reducing the associated risk of brake failure. With the use of auxiliary brakes, increased operating safety is thus obtained in the braking system, since the auxiliary brakes may be used for constant speed keeping purposes and for moderate decelerations, while the vehicle's main brakes are saved for violent brakings provoked by unexpected disturbances in the traffic flow.
When a vehicle's speed is being maintained at a constant level by means of a cruise control system, the occurrence of a downhill slope may cause the vehicle to undesirably accelerate. The cruise control system will attempt to reduce the vehicular speed to the desired speed by reducing the amount of throttle commanded and therefore the amount of fuel delivered to the engine. When the throttle has been reduced to zero percent and the vehicle continues to travel at a speed in excess of the desired speed, current cruise control systems require the driver to deactivate the cruise control prior to activation of the auxiliary brakes. Therefore, if the vehicle is accelerating downhill, the driver must exit cruise control mode in order to slow the vehicle using the auxiliary brakes, thereby inconveniencing the driver. Additionally, some drivers will activate the auxiliary brakes too early and deactivate them too late during the deceleration episode, thus resulting in wasted fuel. There is therefore a need for a cruise control system which will allow application of the auxiliary brakes without exiting cruise control. The present invention is directed toward meeting these needs.